1. Field of the Invention
The present invention relates to an ultrasonic diagnostic apparatus, an ultrasonic diagnostic method and a data processing program for ultrasonic diagnostic apparatus that generate and display a B mode image which is tissue information of an object and a color Doppler image which is motion information of blood and tissues by transmission and reception of ultrasonic waves, and more particularly, to an ultrasonic diagnostic apparatus, an ultrasonic diagnostic method and a data processing program for ultrasonic diagnostic apparatus that correct aliasing of velocities in a color Doppler image.
2. Description of the Related Art
The color Doppler method in ultrasonic diagnosis is a method to irradiate ultrasonic waves in a living body in the same direction plural times and to extract blood flow information such as a velocity, a power and a dispersion of blood flow by Doppler effect.
In the color Doppler method, signals corresponding to slow-moving from tissues are suppressed by firstly applying a high pass filter (HPF) called a moving target indication (MTI) filter to plural ultrasonic reception signals from the same point. Note that, in the case of observing a moving velocity of not a blood flow but a tissue, the MTI filter is not necessary. Next, the reception signals passing through the MTI filter are applied to an autocorrelator. Then, a power, a dispersion and a velocity of a moving object such as a blood flow and the like are calculated from the output signals of the autocorrelator.
Here, when the velocity exceeds the aliasing velocity, aliasing phenomenon occurs. This derives from the fact that the phase obtained for calculation of a power, a dispersion and a velocity can become only a value in the range of −π to +π. That is, when the velocity exceeds the aliasing velocity, the phase is folded from +π to −π. This is a limit under the sampling theorem. In a color Doppler image, colors change when the velocity exceeds the aliasing velocity. Therefore, there is a possibility that the observation becomes difficult and diagnosis of a blood flow direction is failed.
The conventional color Doppler ultrasonic diagnostic apparatus has a function called zero shift. The function changes the phase range of −π to +π into the phase range of 0 to +2π. This allows observing a blood flow velocity in a direction toward an ultrasonic beam up to twice the aliasing velocity. However, the function can be used only under the condition of no blood flow in a direction away from an ultrasonic beam. That is to say, even if the zero shift is performed, an observable velocity range is 2 π.
A staggered pulse method is a method to measure a phase exceeding 2π which is a limit under the sampling theorem (for example, refer to the patent documents 1, 2 and 3). The staggered pulse method is put to practical use in the field of radar, however, still not in the field of ultrasonic diagnostic apparatus. The reason is that phase interference called speckle occurs in an ultrasonic reflection echo and a second-order subtraction of a phase cannot be observed stably.
Further, the technology to expand a velocity dynamic range (an aliasing velocity/a detectable low flow velocity) with keeping a phase detection range 2π by improving low flow velocity detectability is devised (for example, refer to the patent document 4). However, this technology is not to increase an aliasing velocity.
On the other hand, there is unwrap as a method to detect a phase exceeding 2π, by assuming a continuity of phase change.
FIG. 1 is a diagram explaining unwrap processing in the conventional ultrasonic diagnostic apparatus.
In (a) and (b) of FIG. 1, each abscissa denotes a position and each ordinate axis denotes a phase of signal.
The unwrap is a method in which an occurrence of an aliasing is recognized in case where a phase of the current point is a positive phase and a phase of the adjacent point is a negative phase so that a difference between both the phases is approximate 2π, and a value derived by adding 2π to the phase of the adjacent point is regarded as a new phase value of the adjacent point.
Therefore, as shown in FIG. 1 (a), the observed signals of which phases are folded in the range of −π to +π are corrected to signals having a continuity as shown in FIG. 1 (b) by unwrap processing.
However, the unwrap processing can be performed only as one-dimensional processing basically. Therefore, when the unwrap processing is performed on the line in the interval direction, points to which wrong aliasing correction is performed due to a noise may appear. As mentioned above, when wrong aliasing correction is performed, the phase sifts by 2π from each point to which the wrong aliasing correction is performed. For this reason, a two-dimensional image like with codes may be generated.
The method to perform local unwrap processing two-dimensionally using a two-dimensional velocity distribution is devised as a measure to the problem as mentioned above (for example, refer to the patent document 5). The method to perform a local unwrap processing is a method to define points, on each of which a round integration value of gradient of a phase is not 0, containing a point in a two-dimensional space, as residues and to select a route so that the residues are avoided.
However, the conventional method to perform the local unwrap processing two-dimensionally has a problem that it takes a long time to calculate for a complicated processing. Further, there is a problem that the points to avoid the residues become image to be visible awkwardly in the case where the number of the residues is many. Additionally, there is a problem that unwrap may become impossible due to a robustness problem.
[Patent Document 1]
Japanese Publication of Patent Application No. 4-197249
[Patent Document 2]
Japanese Publication of Patent Application No. 4-197250
[Patent Document 3]
Japanese Publication of Patent Application No. 4-278864
[Patent Document 4]
Japanese Publication of Patent Application No. 2005-176997
[Patent Document 5]
United States Patent Application Publication No. US 2007/0066896